The measure would bar parents from seeking vaccine exemptions for their children because of religious or personal beliefs, joining California with Mississippi and West Virginia in such strict requirements. Medical waivers would only be available for children with health problems, forcing unvaccinated children to be homeschooled.
After more than three hours of testimony, supporters postponed a vote until next week so authors could work on revisions to address concerns raised in the Senate Education Committee.
The proposal's lead author, Sen. Richard Pan, a Democratic pediatrician from Sacramento, said he is confident he can clarify his bill so lawmakers are comfortable in supporting his bill.
"Every child deserves an opportunity at education," Pan said, "and every child deserves an opportunity to be safe at school."
The proposal was among several drafted across the nation in the wake of a measles outbreak that started at Disneyland in December, sickening more than 100 people in the U.S. and Mexico.
It's generated such an angry debate that Pan has received added security. In addition to threatening messages sent to his office, opponents of the legislation have posted images online comparing Pan to Adolf Hitler.
Hundreds of people lined the Capitol halls ahead of the Legislature's second hearing, with about 600 opponents testifying another 100 people voicing support. The bill has the backing of the Senate's leading lawmaker as well as broad support from doctors, hospitals, teachers, public health officials, local governments and unions.
Parents have been on both sides of the issue, with some calling the vaccination plan an unconstitutional government overreach and others saying it was necessary to save lives.
Carl Krawitt, of Corte Madera near San Francisco, told lawmakers Wednesday that he feared for his 6-year-old son's life during the measles outbreak because the boy, Rhett, could not be vaccinated while he was treated for leukemia.
"We're here for the community," Krawitt said. He added, "You have a duty to legislate from solid evidence, not from fear, and keep our schools safe."
Opposing parents have told lawmakers that since vaccines come with risks, they should have the choice of whether their children should get such shots. Many said they would rather homeschool their children than comply with a vaccination requirement.
Among the risks, opponents say, vaccine drugs have been linked to autism and other developmental diseases, even as the medical community says such claims have been disproved.
Robert Moxley, a Wyoming attorney who represents families who say they've been injured by vaccines, testified that the proposal would not stand up in a court challenge. Pan said courts have validated public health measures.
Across California, pockets of low-immunization rates have forced communities to spend large amounts of resources to contain outbreaks.
"Recent events such as the measles outbreak that began in Disneyland last December has proved that our community immunity is weighing to dangerously low levels due to an over decade of increasing use of the personal belief exemption in too many schools," Pan said.
Republican Minority Leader Bob Huff said he's concerned that unvaccinated children would be forced into homeschooling, which could deprive them of resources and socialization available in schools.
He said recent outbreaks have not constituted a crisis.
"If we were talking about an Ebola outbreak and we had an inoculation against that, I would probably have a different outcome," Huff said.